This invention relates generally to reducing noise using vibrational inputs and more particularly concerns a method and apparatus for minimizing the noise generated from an aircraft engine by shaking the engine or members attached to the engine to create a noise field which cancels the primary field.
Excessive noise has always been a problem confronting the aviation industry. Noise from discrete tones generated by the fans and turbines of modern aircraft engines is a major source of the noise problems. This is particularly true in communities surrounding airports which are subjected to the noise of take off and landing operations. Lately fan and turbine noise has become an important issue due to the louder fan noise of the large-thrust high bypass engines and the increased enactment and enforcement of strict community anti-noise regulations.
Turbine and fan noise has been addressed for many years using by employing passive techniques such as noise absorbing liners, tuned resonators, or a combination thereof. Such devices are usually mounted at the inlet and outlet of the engine to reduce noise radiated from the engine. Some disadvantages associated with these passive techniques include added weight and decreased thrust. Also, absorbent liners are generally ineffective against the long wavelengths of low frequency noise and tuned resonator are only useful at the frequency to which they are tuned.
One possible active approach to fan and turbine noise control is to cancel the noise using cancelling noise fields generated with acoustical inputs. However, the acoustical inputs, such as loudspeakers, are expensive, heavy, and require a relatively large amount of power. Thus, there remains a need for efficient, low weight, low power, and low cost noise reduction.